Some power tool transmissions are user-configurable to provide different speed outputs. For example, an operator of a multi-speed drilling machine may configure the drilling machine for high speed operation or low speed operation by actuating a mode switch on the drilling machine. However, this requires that the drilling machine be stopped, i.e. the transmission is stopped, in order to actuate the mode switch to effect a change in the speed of operation. It may even require removal of the drilling machine drill bit from the work piece prior to stopping of the drilling machine to effect the change in speed of operation in order to prevent locking or snagging of the drill bit in the work piece.
Other power tools, such as circular saw machines only provide a single speed of operation. The load exerted on the circular saw machine is dependent to some degree on the thickness and type of material being sawed. As such, variability in the speed of operation and thus the torque delivered to the saw blade is desirable. One solution would be to adapt the circular saw machine to have a mode switch of the type often found in drilling machines. However, since such mode switches can only be actuated when the machine is stopped, i.e. the machine's transmission is not operating, this either requires the circular saw machine to be stopped whilst the saw blade is in contact with the work piece or for the work piece to be removed from the circular saw machine. In the first instance, there is a danger when stopping a circular saw machine that is being progressed through a work piece that the saw blade, on stopping in contact with the work piece, will become jammed or snagged in the work piece thereby making restarting the sawing operation difficult. In any event, the lack of continuity in sawing a work piece caused by stopping the saw during its progress through the work piece commonly results in a snitch or discontinuity being created in the saw line through the work piece which is undesirable. In the latter instance, a similar effect can be created when re-introducing a removed work piece to the circular saw machine. In this instance, other problems include realigning the re-introduced work piece in the circular saw machine to continue an interrupted cutting operation.
Furthermore, irrespective of the thickness and type of material being sawed, the load exerted on the circular saw machine is also to some degree dependent on the operator's actions. This is also true of other types of power tools including drilling machines, routers, jig saws, screwdrivers and reciprocating saws. Where an operator of a circular saw machine pushes the saw machine with high force, the loading on the saw machine is greatly increased over that where the operator has a light touch, namely the operator pushes the saw machine forward relatively lightly. This disparity in loading caused by the operator's actions is present even where operators exhibiting the different behaviours mentioned above are sawing identical material of identical thickness.
A further issue affecting the loading exerted on a circular saw machine is disparities in harness of the material being sawed. Again, the same issue affects other types of power tools. In the case of a drilling machine being employed to drill a hole in masonry, for example, the masonry may comprise layers of different hardness material that are encountered in succession. Consequently, even where an operator applies a steady, constant pressure to the drilling machine, the loading on the drill bit and thus on the machine's motor will be dependent to a degree on how hard the layer of material presently being drilled in the masonry is. In the case of a circular saw machine sawing a sheet of soft wood, for example, knots in the wood are relatively harder than the remainder of the wood sheet and result in higher load being exerted on the saw blade and consequently on the motor of the circular saw machine. Whilst a single speed circular saw machine can be successfully used on most occasions to cut through a sheet of wood with knots, there is a tendency for the operator to exert greater force on the saw machine when the saw blade encounters a knot thus greatly increasing the load exerted on the machine's motor through a combination of the two effects of the operator's action in exerting a greater urging force and the change in hardness of the material presently being sawed.
Generally, providing a power tool with a single speed of operation is disadvantageous in that it does not account for disparities in load exerted on the tool's motor through an operator's actions and/or through changes in the characteristics of the work piece being worked, e.g. changes in thickness, nature or type of material being worked. More particularly, providing a power tool with only a low speed, high torque mode of operation will frustrate a user when working with a work piece of a type that could be successfully worked using a high speed, low torque mode of operation. The converse is true when providing a power tool with only a high speed, low torque mode of operation. As already discussed above, one solution is to provide the power tool with more than one speed of operation and a mode selection switch for changing the speed of operation to suit the task in hand. However, operators are apt to always select the highest speed of operation to complete a task as quickly as possible and to persevere with the task in hand at the selected speed of operation even when a mode change would be of benefit. This is because the mode selection switches provided in known power tools require the power tool to be stopped before effecting a change of speed mode and operators are often reluctance to stop and make a speed change. Consequently, even with power tools having a switch mode means for changing the speed of operation, operators are apt to not make use of such switch mode means with the result that it is easy, when operating the power tool at its higher speeds of operation, to overload the tool's motor which can result in burning out of the motor or other damage to the power tool such as damage to a battery of a cordless power tool.